1. Field of the Invention
The present invention relates to semiconductor wafer inspection apparatuses, and more particularly, to a semiconductor wafer inspection apparatus for measuring diameters of particles and counting those particles on a surface of a semiconductor wafer.
2. Description of the Background Art
Conventionally, a semiconductor wafer inspection apparatus has been known that is used for measuring diameter of particles and counting those particles on a surface of a semiconductor wafer according to a distribution of intensity of scattered light with respect to angles of scattering, using the Layleigh-Debye approximation or the like. Such an apparatus is disclosed, for example, in Leon L. Pesotchinsky and Zinovy Fichtenholz: IEEE Transactions Semiconductor Manufacturing, Vol. 1, No. 1, pp. 16-22, 1988.
FIG. 17 is a schematic diagram showing a structure of a conventional semiconductor wafer inspection apparatus. Referring to FIG. 17, in the conventional semiconductor wafer inspection apparatus 700, there is provided a sample placing portion 200 having a stage 220 for scanning a sample (semiconductor wafer) 210 two dimensionally. Also provided is a light projecting portion 100 for generating a beam of light 110 with which a surface of sample 210 is irradiated. A light collecting portion 300 is provided between light projecting portion 100 and sample placing portion 200 for collecting light scattered from the surface of sample 210. Light collecting portion 300 includes a ellipsoid of revolution. A light receiving portion 500 is provided at the focus (F2) of scattered light collected by light collecting portion 300 for measuring a total amount of collected light. Also provided is a measurement control portion 600 for driving light projecting portion 100 and sample placing portion 200 and for processing an output signal supplied from light receiving portion 500.
As for an operation of the conventional semiconductor wafer inspection apparatus 700, firstly, measurement control portion 600 transmits a drive signal to light projecting portion 100 for generating a beam of light 110 from light projecting portion 100. Beam of light 110 passes through an optical beam inlet hole 310 provided at light collecting portion 300 to become an incident light 120. A particle 230 on the surface of sample 210 is irradiated with incident light 120. A point where incident light 120 crosses sample 210 is the focus (F1) of light collecting portion 300. Beams of light scattered by irradiation of particle 230 with incident light 120 are collected to the other focus (F2) by light collecting portion 300. The scattered light collected to the focus (F2) is received by light receiving portion 500, whereby an intensity of scattered light corresponding to particle 230 is measured. Accordingly, a diameter of particle 230 can be measured. The above operation is carried out with stage 220 moved so that measurement and counting of particles 230 on the surface of sample 210 can be carried out.
However, there has been an inconvenience in the above conventional semiconductor wafer inspection apparatus 700 such that a very small pit (hole) on the surface of sample 210 could be identified as a particle by mistake. FIG. 18 is a schematic diagram showing scattering of light caused by a very small pit on the surface of sample 210. FIG. 19 is a graph showing a distribution of intensity of scattered light with respect to angles of scattering in the case when the particle exists on the surface of the sample (P), when a pit exists on the surface of the sample (S), and when neither the particle nor the pit exists on the surface of the sample (N).
Referring to FIG. 19, it is assumed that an intensity of scattered light at a scattering angle .theta.=0 is I.sub.0, and that an intensity of scattered light at the other angles .theta. is I. A normalized log (I/I.sub.0) is plotted corresponding to a light scattering angle .theta.. As shown in FIG. 19, in the range of 0-.theta..sub.s where light scattering angles are small, almost the same distribution of intensity of scattered light with respect to angles of light scattering is obtained when the particle exists (P) and when the pit exists (S) on the surface of sample 210. In such a range of light scattering angles 0-.theta..sub.s, the intensity of scattered light is very much greater than that at larger angles of scattering. In this respect, as shown in FIG. 19, even when there is an apparent difference between (P) and (S) at larger angles of light scattering, ratio of a total amount of scattered light of (P) to that of (S) becomes smaller if the difference between (P) and (S) is negligible in the range of smaller angles (0-.theta..sub.s) of light scattering.
Therefore, it has been difficult to clearly distinguish the scattered light (S) caused by pit 240 from the scattered light (P) caused by particles when measuring the total amount of scattered light (S) when the small pit 240 exists (see FIG. 18) and the total amount of scattered light (P) when particles 230 exist (see FIG. 17) on the surface of sample 210. Accordingly, the scattered light (S) caused by pit 240 could erroneously be identified as scattered light (P) caused by particles. Correct measurement of diameters of particles on the surface of sample 210 and counting the number of those particles have been difficult with the conventional semiconductor wafer inspection apparatus 700.